[dev.boringcrypto] all: convert +build to //go:build lines in boring-specific files

[dev.boringcrypto] all: convert +build to //go:build lines in boring-specific files

This makes the latest Go 1.18 cmd/dist happier.

Change-Id: If1894ce4f60a0b604a2bd889974ddb16c405e0c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/359474
Trust: Russ Cox <rsc@golang.org>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>